Multi-threaded Programming Makes You Crazy?

gduranceau writes "Help! My program deadlocks! I got several concurrent threads that write the same variable! Everything goes well on my mono processor but becomes an incredible mess on that 16 CPU monster! And of course, as soon as I add traces, problems disappear... Don't panic! Calm down and take a deep breath. "

Comments

Use the right tool

[AKAImBatman]

And of course, as soon as I add traces, problems disappear... Don't panic! Calm down and take a deep breathe...

...and get yourself a technology designed for multi-threaded programming. Java will give each thread its own cache of variables to prevent deadlocking on concurrent modifications. If you need to do something that requires more than one statement (thus creating a race condition), then you need to create yourself a semaphore-based lock:

synchronized(objectToModify)
{
    if(objectToModify.getX() == myObj.getX()) objectToModify.setY(myObj.getY());
}

Of course, such synchronizations can carry a huge penalty on multi-CPU systems. i.e. If you manage to stop every CPU, you could be wasting MASSIVE amounts of CPU time. As a result, you should always strive to push locks down as far in the code as possible. They must execute extremely quickly, and should only be called when absolutely necessary. Follow those guidelines and you'll find it fairly easy to write multi-threaded code.

Oh wait. I was supposed to praise the NPTL tool, wasn't I. Um... well... it's very nice. And they've got... um... penguins on the homepage. Oh, and look! It's GPLed! Wow. Just... um... wow. Hey, did you know that the author of Minix wrote a book on OS Design? Really. It even covers the basics of multi-threading. It's pretty cool, you should... um... check it out. Yeah, that's the ticket!